In situ exfoliation method to fabricate a graphene-reinforced polymer matrix composite

ABSTRACT

A method for forming a graphene-reinforced polymer matrix composite is disclosed. The method includes distributing graphite microparticles into a molten thermoplastic polymer phase; and applying a succession of shear strain events to the molten polymer phase so that the molten polymer phase exfoliates the graphite successively with each event until at least 50% of the graphite is exfoliated to form a distribution in the molten polymer phase of single- and multi-layer graphene nanoparticles less than 50 nanometers thick along the c-axis direction.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of U.S. Nonprovisionalapplication Ser. No. 14/437,040, filed on Apr. 20, 2015, now U.S. Pat.No. 9,896,565 issued Feb. 20, 2018, which is the National Phase ofInternational Patent Application Serial No. PCT/US/13/31495, filed Mar.14, 2013, which claims the benefit of priority under 35 U.S.C. § 119(e)of U.S. Provisional Patent Application Ser. No. 61/716,461, filed onOct. 19, 2012, which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

The present invention relates to high efficiency mixing methods totransform a polymer composite containing well-crystallized graphiteparticles into nano-dispersed single or multi-layer graphene particleshaving various commercial applications.

BACKGROUND OF THE INVENTION

Polymer compositions are being increasingly used in a wide range ofareas that have traditionally employed the use of other materials suchas metals. Polymers possess a number of desirable physical properties,are light weight, and inexpensive. In addition, many polymer materialsmay be formed into a number of various shapes and forms and exhibitsignificant flexibility in the forms that they assume, and may be usedas coatings, dispersions, extrusion and molding resins, pastes, powders,and the like.

The various applications for which it would be desirable to use polymercompositions require materials with electrical conductivity. However, asignificant number of polymeric materials fail to be intrinsicallyelectrically or thermally conductive enough for many of theseapplications.

Graphene is a substance composed of pure carbon in which atoms arepositioned in a hexagonal pattern in a densely packed one-atom thicksheet. This structure is the basis for understanding the properties ofmany carbon-based materials, including graphite, large fullerenes,nano-tubes, and the like (e.g., carbon nano-tubes are generally thoughtof as graphene sheets rolled up into nanometer-sized cylinders).Graphene is a single planar sheet of sp² bonded carbon atoms. Grapheneis not an allotrope of carbon because the sheet is of finite size andother elements can be attached at the edge in non-vanishingstoichiometric ratios.

When used to reinforce polymers, graphene in any form increases polymertoughness by inhibiting crack propagation. Graphene is also added topolymers and other compositions to provide electrical and thermalconductivity. The thermal conductivity of graphene makes it an idealadditive for thermal management (e.g., planar heat dissipation) forelectronic devices and lasers. Some commercial applications of carbonfiber-reinforced polymer matrix composites (CF-PMCs) include aircraftand aerospace systems, automotive, electronics, governmentdefense/security, pressure vessels, and reactor chambers, among others.

Progress in the development of low cost methods to effectively producegraphene-reinforced polymer matrix composites (G-PMCs) remains veryslow. Currently, some of the challenges that exist affecting thedevelopment of G-PMCs viable for use in real world applications are thatthe materials used are expensive and the presently used chemical ormechanical manipulations have not been practical for large-scalecommercial production. It would thus be desirable for a low cost methodto produce a G-PMC suitable for large-scale commercial production thatoffers many property advantages, including increased specific stiffnessand strength, enhanced electrical/thermal conductivity, and retention ofoptical transparency.

SUMMARY OF THE INVENTION

The present disclosure provides polymer processing methods to fabricatea graphene-reinforced polymer matrix composite (G-PMC) by elongationalflow and folding of well-crystallized graphite particles dispersed in amolten polymer matrix.

In one aspect, there is provided herein a method for forming agraphene-reinforced polymer matrix composite, including: distributinggraphite microparticles into a molten thermoplastic polymer phase; andapplying a succession of shear strain events to the molten polymer phaseso that the molten polymer phase exfoliates the graphite successivelywith each event until at least 50% of the graphite is exfoliated to forma distribution in the molten polymer phase of single- and multi-layergraphene nano-particles less than 50 nanometers thick along a c-axisdirection.

In certain embodiments, the graphite particles may be prepared bycrushing and grinding a graphite-containing mineral to millimeter-sizeddimensions.

In certain embodiments, the millimeter-sized particles may be reduced tomicron-sized dimensions using ball milling or attritor milling.

In certain embodiments, the graphite particles are extracted from themicron-sized particle mixture by a flotation method.

In certain embodiments, the extracted graphite particles may beincorporated in a polymer matrix using a single screw extruder withaxial fluted extensional mixing elements or spiral fluted extensionalmixing elements.

In certain embodiments, the graphite-containing polymer matrix issubjected to repeated extrusion to induce exfoliation of the graphiticmaterial, thus forming a uniform dispersion of graphene nanoparticles inthe polymer matrix.

In certain embodiments, the polymer is selected from the groupconsisting of polyether-etherketones, polyetherketones, polyphenylenesulfides, polyethylene sulfides, polyetherimides, poly-vinylidenefluorides, polysulfones, polycarbonates, polyphenylene ethers/oxides,nylons, aromatic thermo-plastic polyesters, aromatic polysulfones,thermoplastic polyimides, liquid crystal polymers, thermoplasticelastomers, polyethylenes, polypropylenes, polystyrene,polymethylmethacrylate, polyacrylonitrile, ultra-high-molecular-weightpolyethylene, polytetrafluoroethylene, acrylonitrile butadiene styrene,polycarbonates, polyamides, polyphenylene oxide, polyphenylene sulfide,polyoxymethylene plastic, polyimides, polyaryletherketones,polyetherimide, polyvinylchloride, polyvinylidine fluoride, acrylics,polyphenylene sulfide, polyphenylene oxide, and mixtures thereof.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 50% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 25nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 50% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 90% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 80% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 75% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 70% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thesuccession of shear strain events may be applied until at least 60% ofthe graphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.

In certain embodiments, in combination with other embodiments, thegraphite may be doped with other elements to modify a surface chemistryof the exfoliated graphene nanoparticles.

In certain embodiments, in combination with other embodiments, thegraphite is expanded graphite.

In certain embodiments, in combination with other embodiments, a surfacechemistry or nano structure of the dispersed graphite may be modified toenhance bond strength with the polymer matrix to increase strength andstiffness of the graphene composite.

In certain embodiments, in combination with other embodiments, adirectional alignment of the graphene nanoparticles is used to obtainone-, two- or three-dimensional reinforcement of the polymer matrixphase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating various steps that an in situexfoliation method of fabricating a graphene-reinforced polymer matrixcomposite may implement.

FIG. 2 is a graph illustrating the log shear stress versus the log shearstrain rate collected for a polymer at three different constanttemperatures.

FIG. 3 illustrates the morphology analysis of 2% graphite exfoliated inpolysulfone at mixing times of 3 minutes, 30 minutes, and 90 minutesaccording to an in situ exfoliation method of the present disclosure.

FIG. 4 illustrates micrographs of 90G-PMC at various scales andmagnification levels according to an in situ exfoliation method of thepresent disclosure.

FIG. 5 illustrates a graph of the Debye-Scherrer Equation applied to theaverage XRD results from each 2% graphite exfoliated in polysulfoneaccording to an in situ exfoliation method of the present disclosure.

FIG. 6 illustrates a graph depicting the crystal size versus FWHM of 2%graphite exfoliated in polysulfone according to an in situ exfoliationmethod of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

This disclosure is not limited to the particular systems, methodologiesor protocols described, as these may vary. The terminology used in thisdescription is for the purpose of describing the particular versions orembodiments only, and is not intended to limit the scope.

As used in this document, the singular forms “a,” “an,” and “the”include plural reference unless the context clearly dictates otherwise.Unless defined otherwise, all technical and scientific terms used hereinhave the same meanings as commonly understood by one of ordinary skillin the art. All publications mentioned in this document are incorporatedby reference. All sizes recited in this document are by way of exampleonly, and the invention is not limited to structures having the specificsizes or dimensions recited below. Nothing in this document is to beconstrued as an admission that the embodiments described in thisdocument are not entitled to antedate such disclosure by virtue of priorinvention. As used herein, the term “comprising” means “including, butnot limited to.”

The following term(s) shall have, for purposes of this application, therespective meanings set forth below:

The term “graphene” refers to the name given to a single layer of carbonatoms densely packed into a benzene-ring structure. Graphene, when usedalone, may refer to multi-layer graphene, graphene flakes, grapheneplatelets, and few layer graphene or single layer graphene in a pure anduncontaminated form.

The present invention provides a high efficiency mixing method totransform a polymer composite that contains well-crystallized graphiteparticles into nano-dispersed single or multi-layer graphene particles.The method involves in situ exfoliation of the graphite layers bycompounding in a batch mixer or extruder that impart repetitive, highshear strain rates. In both processes, longer mixing times provideenhanced exfoliation of the graphite into graphene nanoparticles withinthe polymer matrix composite (PMC). In addition, additives may be usedto promote sufficient graphene/polymer bonding, thereby yielding a lowdensity graphene-reinforced polymer matrix composite (G-PMC). The methodis low cost to produce a G-PMC that offers numerous property advantages,including increased specific stiffness and strength, enhancedelectrical/thermal conductivity, and retention of optical transparency.

Repeated compounding during a batch mixing process or single screwextrusion is used to progressively transform the initialgraphite-particle dispersion into a uniform nano-dispersion of discretegraphene nanoparticles. In some cases, an inert gas or vacuum may beused during processing. The method is described herein as “mechanical”exfoliation to distinguish it from “chemical” exfoliation, which is theprimary thrust of much of today's research. An advantage of themechanical method is that contamination-free graphene-polymer interfacesare formed during high-shear mixing, thus ensuring good interfaceadhesion or bonding. Other advantages of in situ exfoliation are that itavoids making and handling graphene flakes, as well as dispersing themuniformly in the polymer matrix phase.

Depending on the number of in situ shear strain events, the methodprovides multi-layer graphene, graphene flakes, graphene platelets, fewlayer graphene or single layer graphene in a pure and uncontaminatedform. Platelets have diamond-like stiffness and are used for polymerreinforcement. Graphene in any form increases polymer toughness byinhibiting crack propagation as a reinforcement for polymers. Graphenemay be used as an additive to polymers and other compositions to provideelectrical and thermal conductivity. The thermal conductivity ofgraphene makes it a desirable additive for thermal management forelectronic devices and lasers.

Graphite, the starting material from which graphene is formed, iscomposed of a layered planar structure in which the carbon atoms in eachlayer are arranged in a hexagonal lattice. The planar layers are definedas having an “a” and a “b” axis, with a “c” axis normal to the planedefined by the “a” and “b” axes. The graphene particles produced by theinventive method have an aspect ratio defined by the “a” or “b” axisdistance divided by the “c” axis distance. Aspect ratio values for theinventive nanoparticles exceed 25:1 and typically range between 50:1 and1000:1.

It should be understood that essentially any polymer inert to graphitecapable of imparting sufficient shear strain to exfoliate graphene fromthe graphite may be used in the method of the present invention.Examples of such polymers include, but are not limited to,polyetheretherketone (PEEK), polyetherketone (PEK), poly-phenylenesulfide (PPS), polyethylene sulfide (PES), polyetherimide (PEI),polyvinylidene fluoride (PVDF), polysulfone (PSU), polycarbonate (PC),polyphenylene ether/oxide, nylons, aromatic thermoplastic polyesters,aromatic polysulfones, thermoplastic polyimides, liquid crystalpolymers, thermoplastic elastomers, polyethylene, polypropylene,polystyrene (PS), polymethylmethacrylate (PMMA), polyacrylonitrile(PAN), ultra-high-molecular-weight polyethylene (UHMWPE),polytetrafluoroethylene (PTFE/Teflon), acrylonitrile butadiene styrene(ABS), polycarbonates (PC), polyamides (PA), polyphenylene oxide (PPO),polyphenylene sulfide (PPS), polyoxymethylene plastic (POM/Acetal),polyimides, polyaryletherketones, polyetherimide (PEI),polyvinylchloride (PVC), polyvinylidine fluoride (PVDF), acrylics,polyphenylene sulfide (PPS), polyphenylene oxide (PPO), mixturesthereof, and the like. Polymers capable of wetting the graphite surfacemay be used as well as high melting point, amorphous polymers inaccordance with the method of the present invention.

The graphene may be produced as a graphene-polymer mixture suitable foruse as-is as a G-PMC that can be pelletized by conventional means forsubsequent fabrication processing. Alternatively, higher concentrationsof graphite may be used at the outset to provide a graphene-polymermasterbatch in concentrated form that can also be pelletized and thenused to add graphene to polymer compositions as a reinforcing agent. Asa further alternative, the graphene may be separated from the polymer,for example, by combustion or selective dissolution, to provideessentially pure particles of graphene.

Graphene-reinforced polymers according to the present inventiontypically contain between about 0.1 and about 30 wt % graphene. Moretypically, the polymers contain between about 1.0 and about 10 wt %graphene. Polymer masterbatches typically contain between about 5 andabout 50 wt % graphene, and more typically between about 10 and about 30wt % graphene.

The availability of graphite-rich mineral deposits, containingrelatively high concentrations (e.g., about 20%) of well-crystallizedgraphite, makes for a low cost and virtually inexhaustible source of rawmaterial. As discussed below, the extraction of graphite particles frommined material can be accomplished in a cost-effective manner. Syntheticgraphite of high purity and exceptional crystallinity (e.g., pyrolyticgraphite) may also be used for the same purpose. However, in this case,the batch mixing or extrusion compounding-induced exfoliation processcreates a laminated composite, in which the graphene nanoparticles areoriented over a relatively large area. Such laminated composites may bepreferred for specific applications.

Mechanical exfoliation of graphite within a polymer matrix may beaccomplished by a polymer processing technique that imparts repetitivehigh shear strain events to mechanically exfoliate graphitemicroparticles into multi- or single-layer graphene nanoparticles withinthe polymer matrix.

For purposes of the present invention, graphite micro-particles aredefined as graphite in which at least 50% of the graphite consists ofmultilayer graphite crystals ranging between 1.0 and 1000 microns thickalong the c-axis of the lattice structure. Typically 75% of the graphiteconsists of crystals ranging between 100 and 750 microns thick. Expandedgraphite may also be used. Expanded graphite is made by forcing thecrystal lattice planes apart in natural flake graphite, thus expandingthe graphite, for example, by immersing flake graphite in an acid bathof chromic acid, then concentrated sulfuric acid. Expanded graphitesuitable for use in the present invention include expanded graphite withopened edges at the bilayer level, such as MESOGRAF.

A succession of shear strain events is defined as subjecting the moltenpolymer to an alternating series of higher and lower shear strain ratesover essentially the same time intervals so that a pulsating series ofhigher and lower shear forces associated with the shear strain rate areapplied to the graphite particles in the molten polymer. Higher andlower shear strain rates are defined as a first, higher, shear strainrate that is at least twice the magnitude of a second, lower shearstrain rate. The first shear strain rate will range between 100 and10,000 sec⁻¹. At least 1,000 to over 10,000,000 alternating pulses ofhigher and lower shear strain pulses are applied to the molten polymerto form the exfoliated graphene nanoparticles. The number of alternatingpulses required to exfoliate graphite particles into graphene particlesmay be dependent on the original graphite particle dimensions at thebeginning of this process, i.e., smaller original graphite particles mayneed a fewer number of alternating pulses to achieve graphene thanlarger original graphite particles. This can be readily determined byone of ordinary skill in the art guided by the present specificationwithout undue experimentation.

After high-shear mixing, the graphene flakes in the molten polymer areuniformly dispersed, randomly oriented, and have high aspect ratio.Orientation of the graphene may be achieved by many different methods.Conventional drawing, rolling, and extrusion methods may be used todirectionally align the graphene within the PMC fiber, filament, ribbon,sheet, or any other long-aspect shape. The method to fabricate andcharacterize a G-PMC is comprised of four main steps and is furtherdescribed below:

1. Extraction of crystalline graphite particles from a mineral source;

2. Incorporation of the extracted graphite particles into a polymermatrix phase and conversion of the graphite-containing polymer into agraphene-reinforced polymer matrix composite (G-PMC) by a highefficiency mixing/exfoliation process;

3. Morphology analysis to determine the extent of mechanical exfoliationand distribution of multi-layer graphene and graphene nanoparticles; and

4. X-ray diffraction analysis to determine multi-layer graphene orgraphene crystal size as a function of mechanical exfoliation.

Highly crystalline graphite may be extracted from graphite ore by amulti-step process, as described below.

1. Crushing: A drilled rod of graphite ore from the mine may be placedin a vice and crushed.

2. Grinding: The crushed graphite ore may be then ground by mortar andpestle.

3. Size Reduction: The ground graphite ore may be placed in a sieve witha 1 mm mesh size and size reduced. Larger pieces that do not passthrough the screen may be ground by mortar and pestle and then sizereduced through the 1 mm mesh size again. Eventually, all of thematerial passed through the 1 mm mesh size to obtain graphite orepowder.

4. Density Separation by Water: The 1 mm sized powder may be placed in acolumn filled with water and agitated until a clear separation formedbetween the more dense portions of the solids and the less denseportions. Graphite is near the density of water (1 g/cm³), while siliconis much more dense (2.33 g/cm³). The uppermost materials are siphonedoff with the water and then dried. The dried powder graphite is referredto as Separated Mineral Graphite (SMG).

In commercial practice, very large crushing and grinding machines areavailable to produce tonnage quantities of mixed powders, from which thegraphite component can be separated by standard floatation methods.

Referring now to FIG. 1, a method according to the present disclosure isdepicted in a flow chart illustrating the various steps that an in situexfoliation method of fabricating a G-PMC may implement. In this method,a polymer that is uniformly blended with micron-sized crystallinegraphite particles is subjected to repeated compounding-elementprocessing during batch mixing or extrusion at a temperature where thepolymer adheres to the graphite particles. Typical polymers have a heatviscosity (without graphite) greater than 100 cps at the compoundingtemperature. The compounding temperature will vary with the polymer andcan range between room temperature (for polymers that are molten at roomtemperature) and 600° C. Typical compounding temperatures will rangebetween 180° C. and 400° C.

In one embodiment, the extrusion compounding elements are as describedin U.S. Pat. No. 6,962,431, the disclosure of which is incorporatedherein by reference, with compounding sections, known as axial flutedextensional mixing elements or spiral fluted extensional mixingelements. The compounding sections act to elongate the flow of thepolymer and graphite, followed by repeated folding and stretching of thematerial. This results in superior distributive mixing, which in turn,causes progressive exfoliation of the graphite particles into discretegraphene nanoparticles. Batch mixers may also be equipped withequivalent mixing elements.

Thus, the effect of each compounding pass is to shear-off graphenelayers one after the other, such that the original graphite particlesare gradually transformed into a very large number of graphenenanoparticles. After an appropriate number of such passes, the finalresult is a uniform dispersion of discrete graphene nanoparticles in thepolymer matrix phase. Longer mixing times or a larger number of passesthrough the compounding elements provide smaller graphite crystal sizeand enhanced exfoliation of graphite into graphene nanoparticles withinthe polymer matrix, however, the shear events should not be of aduration that would degrade the polymer.

As the density of graphene nanoparticles increases during multi-passextrusion, the viscosity of the polymer matrix increases, due to theinfluence of the growing number of polymer/graphene interfaces. Toensure continued refinement of the composite structure, the extrusionparameters are adjusted to compensate for the higher viscosity of thecomposite.

Automated extrusion systems are available to subject the compositematerial to as many passes as desired, with mixing elements as describedin U.S. Pat. No. 6,962,431, and equipped with a re-circulating stream todirect flow back to the extruder input. Since processing of thegraphene-reinforced PMC is direct and involves no handling of grapheneparticles, fabrication costs are low.

In order to mechanically exfoliate graphite into multi-layer grapheneand/or graphene, the shear strain rate generated in the polymer duringprocessing must cause a shear stress in the graphite particles greaterthan the critical stress required to separate two layers of graphite, orthe interlayer shear strength (ISS). The shear strain rate within thepolymer is controlled by the type of polymer and the processingparameters, including the geometry of the mixer, processing temperature,and revolutions per minute (RPM).

The required processing temperature and RPM for a particular polymer isdeterminable from polymer rheology data given that, at a constanttemperature, the shear strain rate ({dot over (γ)}) is linearlydependent upon RPM, as in Equation 1. The geometry of the mixer appearsas the rotor radius, r, and the space between the rotor and the barrel,Δr.

$\begin{matrix}{\overset{.}{\gamma} = {\left( \frac{2\;\pi\; r}{\Delta\; r} \right)\left( \frac{RPM}{60} \right)}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

Polymer rheology data collected for a particular polymer at threedifferent temperatures provides a log shear stress versus log shearstrain rate graph, as illustrated in FIG. 2. The ISS of graphite rangesbetween 0.2 MPa-7 GPa, but a new method has quantified the ISS at 0.14GPa. Thus, to mechanically exfoliate graphite in a polymer matrix duringprocessing, the required processing temperature, shear strain rate, andRPM is determinable for a particular polymer from FIG. 2 so that theshear stress within the polymer is equal to or greater than the ISS ofgraphite. Under typical processing conditions, polymers have sufficientsurface energy to behave like the sticky side of scotch tape, and thusare able to share the shear stress between the polymer melt and thegraphite particles.

In one embodiment, a method for forming a G-PMC includes distributinggraphite microparticles into a molten thermoplastic polymer phase. Asuccession of shear strain events are then applied to the molten polymerphase so that the molten polymer phase exfoliates the graphitesuccessively with each event until at least 50% of the graphite isexfoliated to form a distribution in the molten polymer phase of single-and multi-layer graphene nanoparticles less than 50 nanometers thickalong a c-axis direction.

In certain embodiments, the graphite particles may be prepared bycrushing and grinding a graphite-containing mineral to millimeter-sizeddimensions. The millimeter-sized particles may be reduced tomicron-sized dimensions using ball milling and attritor milling.

In certain embodiments, the graphite particles may be extracted from themicron-sized particle mixture by a flotation method. The extractedgraphite particles may be incorporated in a polymer matrix using asingle screw extruder with axial fluted extensional mixing elements orspiral fluted extensional mixing elements. The graphite-containingpolymer matrix is subjected to repeated extrusion as described herein toinduce exfoliation of the graphitic material, thus forming a uniformdispersion of graphene nanoparticles in the polymer matrix.

In other embodiments, the succession of shear strain events may beapplied until at least 50% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 90% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 80% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 75% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 70% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the succession of shear strain events may beapplied until at least 60% of the graphite is exfoliated to form adistribution in the molten polymer phase of single- and multi-layergraphene nanoparticles less than 10 nanometers thick along the c-axisdirection.

In other embodiments, the graphite may be doped with other elements tomodify the surface chemistry of the exfoliated graphene nanoparticles.The graphite is expanded graphite.

In other embodiments, the surface chemistry or nano structure of thedispersed graphite may be modified to enhance bond strength with thepolymer matrix to increase strength and stiffness of the graphenecomposite.

In other embodiments, directional alignment of the graphenenanoparticles is used to obtain one-, two- or three-dimensionalreinforcement of the polymer matrix phase.

EXAMPLE

The present invention is further illustrated by the following example,which should not be construed as limiting in any way. While someembodiments have been illustrated and described, it should be understoodthat changes and modifications can be made therein in accordance withordinary skill in the art without departing from the invention in itsbroader aspects as defined in the following claims.

In one embodiment, a small scale extension mixer with a 10 gram capacitywas used to compound 2% of SMG with Udel P-1700 Polysulfone (PSU) at332° C. (630° F.) and under vacuum for 3, 30, and 90 minutes. The methodis described below. Samples collected for characterization after eachlength of time are referred to as 3G-PMC, 30G-PMC, 90G-PMC.

1. 9.8 grams of PSU were added to the mixer and allowed to becomemolten.

2. 0.2 grams of SMG were added to the molten PSU and mixed.

3. After 3 minutes of mixing time, 3 grams of the G-PMC were extrudedout of the mixer and collected for characterization.

4. 3 grams of 2% SMG in PSU was added to the mixer and mixed.

5. After 30 minutes of mixing time, 3 grams of the G-PMC were extrudedout of the mixer and collected for characterization.

6. 3 grams of 2% SMG in PSU was added to the mixer and mixed.

7. After 90 minutes of mixing time, 3 grams of the G-PMC were extrudedout of the mixer and collected for characterization.

Morphology Analysis

A Zeiss Sigma Field Emission Scanning Electron Microscope (FESEM) withOxford EDS was used to determine the degree of mechanical exfoliation ofgraphite into multi-layer graphene or graphene nanoparticles and thethickness of these particles. An accelerating voltage of 3 kV andworking distance of approximately 8.5 mm was used during viewing. Priorto viewing, specimens from each sample of 3G-PMC, 30G-PMC, and 90G-PMCwere notched, cryogenically fractured to produce a flat fracturesurface, placed under vacuum for at least 24 hours, gold coated, andstored under vacuum.

X-ray Diffraction Analysis (XRD)

XRD analysis on each sample of 3G-PMC, 30G-PMC, and 90G-PMC includesfour steps: (1) sample preparation, (2) diffraction pattern acquisition,(3) profile fitting, and (4) out-of-plane (D) crystallite sizescalculation according to the Debye-Scherrer equation.

1. The samples for XRD analysis were prepared by pressing thin films ofeach sample 3G-PMC, 30G-PMC, and 90G-PMC at 230° C. and 5,500 psi over a2 minute time period. Each sample was positioned between aluminum sheetsprior to pressing using a Carver Uniaxial Press with heated platens.

2. Diffraction patterns of the pressed films were acquired using aPhilips XPert powder Diffractometer with sample changer (Xpert) at 40 kVand 45 mA with an incident slit thickness of 0.3 mm from 4°-70° 2θ and astep size of 0.02° 2θ.

3. Diffraction patterns were uploaded into WinPLOTR Powder diffractiongraphics tool, without background editing or profile adjustments priorto peak fitting. Single peak fitting was applied at a 2θ range of26°-27.5°, using a pseudo-Voigt function and taking into account aglobal FWHM, global eta (proportion of Lorentz), and linear background.Single peak fitting of the profile provides the full width at halfmaximum (FWHM) of the relevant peak.

4. The average out of-plane crystallite size (D) (sometimes referred toas along the c-axis, and proportional to the number of graphene layerswhich are stacked) is calculated using the Debye-Scherrer Equation andthe (002) FWHM values, for which λ is the X-ray wavelength, coefficientK=0.89, β is the FWHM in radians, and θ is the diffraction angle. Thed-spacing is also calculated.

$\begin{matrix}{D = \frac{K\;\lambda}{\beta cos\theta}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

Morphology Results

The morphology of each sample, 3G-PMC, 30G-PMC, and 90G-PMC, at threedifferent scales (magnification) is shown in FIG. 3. In (a-c), a 20 μmscale and 1,000× magnification shows good distribution of multi-layergraphene or graphene within the PSU matrix at each mixing time. In(d-f), a 1 μm scale and 10,000× magnification and (g-i), a 1 μm scaleand 50,000× magnification shows mechanically exfoliated graphite withinthe PSU matrix. In (d-i), micro-folding of the multi-layer graphene orgraphene is evident, as well as good bonding between the graphenenanoparticles and the polymer matrix.

The 90G-PMC sample, the sample mixed for the longest time and exposed tothe most repetitive shearing, exhibits superior mechanical exfoliationand the smallest crystal size. As shown in FIG. 4, mechanicalexfoliation has reduced the graphene nanoparticle thickness in the90G-PMC sample to 8.29 nm.

X-ray Diffraction Results

The Debye-Scherrer equation was applied to the FWHM and d-spacingresults obtained from the X-ray diffraction patterns for 3G-PMC,30G-PMC, and 90G-PMC to provide the crystal thickness (D) of themulti-layer graphene or graphene nanoparticles. The XRD results andcrystal thickness appear in Table 1. For the 3G-PMC, 30G-PMC, and90G-PMC samples, the crystal thickness is 40 nm, 31 nm, and 23 nm; theFWHM is 0.202°, 0.257°, and 0.353°; and the d-spacing is 3.361 nm, 3.353nm, and 3.387 nm, respectively. The FWHM increases with mixing time, andcrystal thickness decreases with mixing time (FIG. 5), which indicatesthat mechanical exfoliation of the graphite to multi-layer graphene orgraphene is occurring and is enhanced over longer mixing times. FIG. 6shows the decrease in crystal size as a function of FWHM.

TABLE 1 Debye-Scherrer Equation applied to the average XRD results fromeach 2% Graphite Exfoliated in PSU sample mixed for 3 min, 30 min, and90 min Mixing Average D - Time (d 002) FWHM Crystal Thickness (nm)Sample (min) (nm) (degrees) Along c-Axis Direction 3G-PMC 3 0.3361 0.20240 30G-PMC 30 0.3353 0.257 31 90G-PMC 90 0.3387 0.353 23

Graphene Modification

Mechanical exfoliation of the graphite into multi-layer graphene orgraphene as a result of the repetitive shear strain action in thepolymer processing equipment generates dangling primary and secondarybonds that provide the opportunity for various chemical reactions tooccur, which can be exploited to obtain property enhancement of theG-PMC. This represents an advance over prior art conventional methodsforming graphene oxides, where the dangling primary and secondary bondscovalently bond with oxygen, which typically remain in these positionseven after the graphene oxide is reduced.

For example, chemical reactions that covalently attach these danglingbonds from the multi-layer graphene or graphene nanoparticles to thepolymer matrix would provide superior mechanical properties of theG-PMC. Alternatively, electrical conductivity may be enhanced bychemically linking appropriate band gap materials at the graphenenano-particle edges or by coordinating with conductive metals such asgold, silver, copper, and the like. The graphene-reinforced polymer maythen be added to polymers or other compositions to provide or increaseelectrical conductivity. The bonds may also be coordinated to metals,such as platinum and palladium, to provide a catalyst, with thegraphene-reinforced polymer serving as a catalyst support. Other formsof functionalized graphene are disclosed in U.S. Pat. No. 8,096,353, thedisclosure of which is incorporated herein by reference.

The method of the present invention is particularly advantageous becausein situ functionalization reactions may be performed during theexfoliation process via one pot reactive compounding.

The graphene-reinforced polymers may be used as electrodes forlightweight batteries. Other uses include composite boat hulls,aircraft, aerospace systems, transportation vehicles, personnel armor,pressure vessels, reactor chambers, spray coatings, polymer powders for3-D printing, transparent electrodes for electronic device touchscreens, and the like. Addition of 1-2 wt % graphene to a polymer matriximparts electrical conductivity, while maintaining optical transparency,thus enabling applications in solar panels, flat-panel displays, and forstatic-discharge control in hospitals.

Mechanical exfoliation successfully converted 2% graphite melt-blendedwith PSU into a G-PMC using a repetitive shearing action in theRandcastle Extrusion System's Small Scale Extension Mixer. Results maybe improved by machine modification to increase shear; for example, byusing a larger diameter mixing element to increase rotational speedand/or by minimizing the spacing between the mixing element and thecylinder wall.

1. Modified Randcastle Extrusion System's Small Scale Extension Mixer:

The design of the existing small batch mixer may be modified to providehigher shear rate, which in turn provides superior mechanicalexfoliation of graphite within the polymer matrix. The shear rate, {dotover (γ)}, is calculated according to Equation 3, where r is the toolingradius and Δr is the clearance for compounding. Machine modificationsare listed in Table 2, along with the maximum achievable shear rate. Thenewly designed mixer has a maximum shear rate 22 times that of thecurrent mixer, which will provide enhanced mechanical exfoliation ofgraphite within a polymer matrix at shorter lengths of time. In otherwords, the crystal size, D, may be reduced to smaller dimensions in amore efficient length of time.

$\begin{matrix}{\overset{.}{\gamma} = {\left( \frac{RPM}{60} \right)\left( \frac{2\pi\; r}{\Delta\; r} \right)}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

TABLE 2 Modifications of the Randcastle Extrusion System's Small ScaleExtension Mixer to provide enhanced mechanical exfoliation CurrentImproved Randcastle Randcastle Mixer Mixer Tooling Radius (inches) 0.5 1Clearance for Compounding, Δr (in) 0.04 0.01 Maximum RPM 100 360 MaximumShear Strain Rate (sec⁻¹) 133 2900

2. Modified Single Screw Extrusion:

Randcastle has made modifications to the extruder screw that will betterenable mechanical exfoliation of the graphite into multi-layer grapheneor graphene in a polymer matrix to fabricate a G-PMC.

The foregoing examples and description of the preferred embodimentsshould be taken as illustrating, rather than as limiting the presentinvention as defined by the claims. As will be readily appreciated,numerous variations and combinations of the features set forth above canbe utilized without departing from the present invention as set forth inthe claims. Such variations are not regarded as a departure from thespirit and scope of the invention, and all such variations are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for forming a graphene-reinforcedpolymer matrix composite, comprising: (a) distributing graphitemicroparticles into a molten thermoplastic polymer phase, wherein atleast 50% by weight of the graphite in the graphite microparticlesconsists of multilayer graphite crystals between 1.0 and 1000 micronsthick along a c-axis direction; and (b) applying a succession of shearstrain events to the molten polymer phase in situ so that the shearstress within said molten polymer phase is equal to or greater than theInterlayer Shear Strength (ISS) of said graphite microparticles and saidmolten polymer phase mechanically exfoliates the graphite successivelywith each event until said graphite is at least partially exfoliated toform a distribution in the molten polymer phase of essentially pure anduncontaminated single- and multi-layer graphene nanoparticles less than10 nanometers thick along the c-axis direction.
 2. The method of claim1, wherein the graphite particles are prepared by crushing and grindinga graphite-containing mineral to millimeter-sized dimensions.
 3. Themethod of claim 2, wherein the millimeter-sized particles are reduced tomicron-sized dimensions using ball milling and attritor milling.
 4. Themethod of claim 3, wherein the graphite particles are extracted from themicron-sized particle mixture by a flotation method.
 5. The method ofclaim 4, wherein the extracted graphite particles are incorporated in apolymer matrix using a single screw extruder with axial flutedextensional mixing elements or spiral fluted extensional mixingelements.
 6. The method of claim 5, wherein the graphite-containingpolymer matrix is subjected to repeated extrusion to induce exfoliationof the graphitic material, thus forming a uniform dispersion of graphenenanoparticles in the polymer matrix.
 7. The method of claim 6, whereinthe polymer is selected from the group consisting ofpolyether-etherketones, polyetherketones, polyphenylene sulfides,poly-ethylene sulfides, polyetherimides, polyvinylidene fluorides,polysulfones, polycarbonates, poly-phenylene ethers/oxides, nylons,aromatic thermoplastic polyesters, aromatic polysulfones, thermoplasticpolyimides, liquid crystal polymers, thermoplastic elastomers,polyethylenes, polypropylenes, polystyrene, polymethylmethacrylate,polyacrylonitrile, ultra-high-molecular-weight polyethylene,polytetrafluoroethylene, acrylonitrile butadiene styrene, polyamides,poly-phenylene oxide, polyoxymethylene plastic, polyimides,polyaryletherketones, polyvinylchloride, acrylics, and mixtures of twoor more thereof.
 8. The method of claim 1, wherein the succession ofshear strain events is applied until at least 50% by weight of thegraphite is exfoliated to form a distribution in the molten polymerphase of single- and multi-layer graphene nanoparticles less than 10nanometers thick along the c-axis direction.
 9. The method of claim 1,wherein the succession of shear strain events is applied until at least90% by weight of the graphite is exfoliated to form a distribution inthe molten polymer phase of single- and multi-layer graphenenanoparticles less than 10 nanometers thick along the c-axis direction.10. The method of claim 1, wherein the succession of shear strain eventsis applied until at least 75% by weight of the graphite is exfoliated toform a distribution in the molten polymer phase of single- andmulti-layer graphene nanoparticles less than 10 nanometers thick alongthe c-axis direction.
 11. The method of claim 1, wherein the graphite isdoped with other elements to modify a surface chemistry of theexfoliated graphene nanoparticles.
 12. The method of claim 1, whereinthe graphite is expanded graphite.
 13. The method of claim 1, wherein asurface chemistry or nanostructure of the dispersed graphite is modifiedto enhance bond strength with the polymer matrix to increase strengthand stiffness of the graphene composite.
 14. The method of claim 1,wherein the graphene nanoparticles are directionally aligned therebyproviding one-, two- or three-dimensional reinforcement of the polymermatrix phase.
 15. The method of claim 1, wherein saidgraphene-reinforced polymer matrix composite contains residual graphitemicroparticles.
 16. A graphene-reinforced polymer matrix compositeprepared by the method of claim 1, wherein the composite comprisescontamination-free graphene-polymer interfaces and wherein the polymeradheres to or is covalently bonded to the graphene-polymer interfaces.17. The graphene-reinforced polymer matrix composite of claim 16,wherein said composite contains between about 0.1% and about 30% byweight of graphene.
 18. The graphene-reinforced polymer matrix compositeof claim 16, wherein said composite contains between about 1% and about10% by weight of graphene.
 19. The graphene-reinforced polymer matrixcomposite of claim 16, wherein said composite contains between about 5%and about 50% by weight of graphene.
 20. The graphene-reinforced polymermatrix composite of claim 16, wherein said composite contains betweenabout 10% and about 30% by weight of graphene.
 21. Thegraphene-reinforced polymer matrix composite of claim 16, wherein thepolymer is selected from the group consisting of polyether-etherketones,polyetherketones, poly-phenylene sulfides, polyethylene sulfides,polyetherimides, polyvinylidene fluorides, polysulfones, polycarbonates,polyphenylene ethers/oxides, nylons, aromatic thermoplastic polyesters,aromatic polysulfones, thermoplastic polyimides, liquid crystalpolymers, thermoplastic elastomers, poly-ethylenes, polypropylenes,polystyrene, polymethylmethacrylate, polyacrylonitrile,ultra-high-molecular-weight polyethylene, polytetrafluoroethylene,acrylonitrile butadiene styrene, poly-amides, polyphenylene oxide,polyoxymethylene plastic, polyimides, polyaryletherketones,polyvinylchloride, acrylics, and mixtures thereof.
 22. Thegraphene-reinforced polymer matrix composite of claim 16 comprisingresidual graphite microparticles.
 23. The graphene-reinforced polymermatrix composite of claim 16, wherein the graphite is expanded graphite.24. The graphene-reinforced polymer matrix composite of claim 16,wherein a surface chemistry or nano structure of the dispersed graphiteis modified to enhance bond strength with the polymer matrix to increasestrength and stiffness of the graphene composite.